Field of the Invention
The invention relates to compositions and methods for increasing the permeability of a subterranean formation. More particularly, the invention relates to acidizing compositions and methods for their application in subterranean formations comprising siliceous material.
Background
Acidizing of siliceous formations by injection of compositions referred to as mud acid is common practice in oilfield operations. As commonly understood, the expression xe2x80x9cmud acidxe2x80x9d refers to an aqueous base mixture formed by blending hydrofluoric acid (HF) and at least one of hydrochloric acid (HCl), acetic acid (C2H4O2), or formic acid (CH2O2), the most common mixture employed being formed from HF and HCl. Often, if acetic acid or formic acid are the acids combined with the HF, the mud acid is referred to as xe2x80x9corganic mud acidxe2x80x9d. As is well recognized in the art, the origin of the species in the acidizing solution or mixture is not critical, so that a xe2x80x9cmud acidxe2x80x9d, which might be formed by directly blending, e.g., HF and HCl, also is understood to include aqueous mixtures or solutions formed by mixing components which quickly react to form the desired ionic species in the solution or mixture. The ratios and amounts of the acids combined may vary over wide ranges, with the lower limits being more a matter of practicality rather than operability, and the upper limits being a matter of mutual solubility of the acids. Most typically, a mud acid is formed by combining about 3 to about 25 percent HCl and about 1 to about 10 percent HF, both percentages by weight, in aqueous solution, and is typically substantially free of other acidic species. As will be recognized by those skilled in the art, mud acids may also contain, and commonly do, one or more functional additives, such as inhibitors, diverting agents, and/or surfactants.
Although conventional treatments of siliceous clay containing formations with mud acids have generally proven effective for a short time, the improvements in production are frequently short lived. One explanation for this phenomenon is that the mud acid reacts rapidly with the subterranean formation in the vicinity of or near wellbore area, usually the first few inches around the wellbore, thus spending so rapidly that penetration deep into the subterranean formation is not achieved. Subsequently, fines in the subterranean formation migrate into the acidized near wellbore area and replug the area.
One solution to this problem is that taught in U.S. Pat. No. 3,828,854 (Templeton et al) and in the xe2x80x9cIntroductionxe2x80x9d section of Society of Petroleum Engineers Paper No. 5153. The approach taken is the provision, down the wellbore, of a composition or solution which generates HF slowly, so that the solution is placed in contact with the subterranean formation before a significant amount of the HF is generated. The composition is a relatively high pH aqueous solution of a water soluble fluoride salt and at least one water reactive organic acid ester.
U.S. Pat. No. 2,300,393 (Ayers, Jr.) discloses treatment of subterranean formations with fluoboric acid, optionally containing small amounts of HF. Ayers, Jr. also teaches that the fluoboric acid treatment may be followed by HCl containing an inappreciable amount of hydrofluoric acid, or optionally, by a mixture of HCl and fluoboric acids. Again, U.S. Pat. No. 2,425,415 (Bond et al.) describes an acidizing procedure in which the subterranean formation is first contacted with a fluoboric acid solution which does not contain free HF, but which contains an excess of boric acid, followed by contact of the subterranean formation with aqueous fluoboric acid containing excess HF. U.S. Pat. No. 2,663,689 (Kingston el. al.) describes the use of boric acid in aqueous HCl-HF to avoid precipitation of insoluble fluoride salts and fluorosilicic acid. U.S. Pat. No. 4,151,878 (Thomas) is directed to the use of a conventional mud acidizing solution (HClxe2x80x94HF), followed by fluoboric acid solution. The use of fluoboric acid as an overflush is believed to deter clay migration and thereby significantly reduce or delay production decline which is often otherwise encountered shortly after conventional mud acidizing treatments.
The Thomas patent also describes injection of a fluoboric acid solution, followed by mud acid (HClxe2x80x94HF) solution. According to the patent, the technique may be used in formations which have a tendency to plug initially upon contact with mud acid, or with HCl commonly used as a preflush ahead of mud acid. When contacted initially with fluoboric acid, such subterranean formations show little or no plugging effects when subsequently treated with mud acid.
However, the Thomas patent does not specifically address formations containing zeolites and chlorites. As will be recognized by those skilled in the art, the use of traditional mud acid is not advisable in subterranean formations which comprise or contain HCl-sensitive materials, e.g., zeolite and chlorites. Additionally, fluoride in the mud acid is believed to bind with aluminum in the subterranean formation and promote deposition of hydrated silica, thereby causing plugging. For example, severe, damaging precipitation of aluminum fluorides during the HF reactions was discovered with formic-HF and acetic-HF fluid systems. See, C. E. Shuchart, et al., xe2x80x9cImproved Success in Acid Stimulations with a New Organic-HF System,xe2x80x9d SPE 36907 presented at 1996 European Petroleum Conference, Milan, Italy. To overcome this problem, Rogers et al. disclosed the use of citric acid as a chelating agent for aluminum to prevent such deposition or formation of hydrated silica gel. The optimum treatment formulation identified therein consisted of 10 percent citric acid and 1.5 percent HF acid, with no additives except corrosion inhibitor. One important disadvantage of this particular method is that the use of hydrofluoric acid primarily addresses damage or scaling in the initial few inches of the subterranean formation around the wellbore, as previously indicated.
Accordingly, there has been a need to extend acidization or stimulation treatment to deeper depths in the formation, e.g., up to a 3 to 5 feet radius from the wellbore, to avoid a rapid decline in production by stabilizing fines and precipitation of acidization products near the wellbore. The invention addresses this need.
The present invention relates to novel acidic compositions useful in treating a well, and to methods for increasing the permeability of a subterranean formation utilizing the compositions as preflush, main, or postflush treatments for the formation. In one embodiment, the invention comprises an aqueous acidic solution or mixture formed by blending an aqueous liquid; fluoboric acid; and an acid, or mixture of acids, which sequester or chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof. In a further embodiment, the composition comprises an aqueous acidic solution or mixture formed or produced by blending an aqueous liquid; a fluoride ion source, as defined, or HF; a boron source; and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof. As utilized herein, the expression xe2x80x9cfluoride ion sourcexe2x80x9d is taken as referring to a compound or compounds, other than HF, or aqueous solutions of the compound or compounds, that will provide fluoride ion or ions in an aqueous liquid. Similarly, the term xe2x80x9cboron sourcexe2x80x9d is considered to define a compound or compounds, or an aqueous solution of the compound or compounds, providing boron ions or boron-containing anions which are reactive with an aqueous liquid or a component in the aqueous liquid to form the BF4xe2x88x92 anion in the aqueous liquid.
In a principal embodiment, therefore, the invention relates to a composition useful for treating a subterranean formation comprising an aqueous acidic solution or mixture formed by blending an aqueous liquid; a fluoride ion source; a boron source; and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof, and to a method of treating a subterranean formation utilizing the composition. In a preferred aspect of this embodiment, the fluoride ion source is selected from ammonium bifluoride and ammonium fluoride, and mixtures thereof, the boron source is boric acid, and the acid which chelates aluminum ions and aluminum fluoride species is selected from polycarboxylic acids, polyaminopolycarboxylic acids, and monoaminopolycarboxylic acids. In one very preferred aspect of this embodiment, the fluoride ion source is ammonium bifluoride, the boron source is boric acid, and the acid which chelates aluminum ions and aluminum fluoride species is selected from citric acid, malic acid, 2-hydroxyethyliminodiacetic acid, N-(2-hydroxyethyl)ethylenediaminetriacetic acid, and mixtures thereof.
In another embodiment of the invention, the composition is formed by blending an aqueous liquid; HCl in specified amount; a fluoride ion source; a boron source; and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof, and to a method of treating a subterranean formation utilizing the composition. Preferably, the fluoride ion source is selected from ammonium bifluoride and ammonium fluoride, and mixtures thereof, the boron ion source is boric acid, and the acid which chelates aluminum ions and aluminum fluoride species is selected from polycarboxylic acids, polyaminopolycarboxylic acids, and monoaminopolycarboxylic acids. In one very preferred aspect, the fluoride ion source is ammonium bifluoride, the boron ion source is boric acid, and the acid which chelates aluminum ions or aluminum fluoride species is selected from citric acid, malic acid, 2-hydroxyethyliminodiacetic acid, N-(2-hydroxyethyl)ethylenediaminetriacetic acid, and mixtures thereof.
In a further embodiment, the invention relates to a composition useful for treating a subterranean formation comprising an aqueous acidic solution or mixture formed by blending an aqueous liquid; HF; a boron source; and a compound or compounds which chelate aluminum ions and aluminum fluoride species, and to a method of treating a subterranean formation utilizing the composition. Preferably, the boron source is boric acid, and the compound(s) which chelate aluminum ions or aluminum fluoride species are selected from polycarboxylic acids, polyaminopolycarboxylic acids, and monoaminopolycarboxylic acids. In one very preferred aspect of this embodiment, the boron source is boric acid, and the compound which chelates aluminum ions or aluminum fluoride species is selected from citric acid, malic acid, 2-hydroxyethyliminodiacetic acid N-(2-hydroxyethyl)ethylenediaminetriacetic acid and mixtures thereof.
In their most preferred aspects, the acid treatment compositions of the invention also include non-interfering ionic species in a concentration or concentrations in the aqueous mixture effective to provide a level, or increase the ionic strength of the composition to a level, sufficient to inhibit migration of clay particles in a subterranean formation when the aqueous mixture is applied to or injected into the subterranean formation. The non-interfering ionic species may be derived from precursor compositions employed in preparing the compositions of the invention, or may be provided by addition of a non-interfering soluble salt or salts during formulation of the compositions of the invention. As understood herein, the term xe2x80x9cnon-interferingxe2x80x9d, in referring to the ionic species, simply indicates that the ionic species do not interfere to any significant extent with the formulation of, or desired treatment function(s) of the compositions of the invention, while the term xe2x80x9csolublexe2x80x9d indicates that any compound or compounds added, or non-interfering species present in the aqueous mixtures, have sufficient solubility in the aqueous mixture to provide the desired concentration level. Preferably, the total ionic strength of the compositions of the invention will range from 2 percent to 10 percent, most preferably from 3 percent to 7 percent, all percentages by weight. Thus, in the case where the invention comprises an aqueous solution or mixture formed by blending an aqueous liquid; fluoboric acid; and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof, ionic species, such as may be provided by addition of at least one non-interfering soluble salt, e.g., NH4Cl, or KCl, may be present in a concentration effective to provide an ionic strength of the invention composition sufficient to inhibit clay particle migration. Where the invention composition is formed by blending an aqueous liquid; a fluoride ion source; a boron source; and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof, the concentration of non-interfering soluble ionic species may be derived sufficiently from the compositions or precursors utilized to formulate the compositions of the invention, or appropriate salt(s) may be added in a concentration effective to increase the ionic strength of the composition to a level sufficient to inhibit clay particle migration. In the case where the composition is formed by blending an aqueous liquid; HCl in specified amount; a fluoride ion source; a boron source and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof, non-interfering soluble salt species may be provided or added to the mixture in a concentration effective to insure that the ionic strength of the composition is at a level sufficient to inhibit clay particle migration. This will also be the case where the compositions of the invention are formed by blending an aqueous liquid; HF; a boron source; and an acid, or mixture of acids, which chelate aluminum ions and aluminum fluoride species, or an ammonium or potassium salt or salts of such acids, or a mixture thereof.
An important aspect of the invention is the requirement of low pH for the compositions of and method of the invention, i.e., the compositions are acidic and are employed as such. The compositions will thus be blended with at least the components specified, but may also be formulated by further addition of an amount of a non-interfering acid, or acids, sufficient to insure pH levels in the acid range, preferably strong acid range. Low pH is considered to aid in maintaining Si and Si species in solution.
As indicated, the invention further includes the use of each composition disclosed in a method for treating a subterranean formation to increase the permeability thereof. The terms xe2x80x9ctreatingxe2x80x9d or xe2x80x9ctreatmentxe2x80x9d are taken herein, as indicated, to include preflush, main or acidizing, or postflush treatments, permeability increase being achieved by reaction with or dissolution of components of the formation. In treating a subterranean formation, the compositions are commonly injected into the formation at a pressure referred to as matrix pressure and allowed to react with or dissolve the minerals or components composing the formation, in amount sufficient or effective to increase the permeability of the formation.